The trend to continue to miniaturize semiconductor integrated circuits to achieve submicron feature sizes and increase the number of devices fabricated on the integrated circuit has required smaller isolation areas between devices. Active areas in which devices are built are isolated by a layer of oxide known as field oxide. The introduction of LOCOS, local oxidation of silicon, produces regions of insulating silicon dioxide between devices. The LOCOS process was a great technological improvement in reducing the area needed for the isolation regions and decreasing some parasitic capacitances.
In LOCOS, silicon nitride is deposited and patterned over a stress relief pad oxide layer. The silicon nitride layer is retained over the area over which further oxidation is not desired. Thus, the silicon nitride is etched to expose a portion of the pad oxide where the field oxide is to be grown. After the thermal oxidation of the exposed pad oxide to form the field oxide regions, the silicon nitride layer is removed.
Several problems occurred, however, with LOCOS. Non-uniform thermal oxidation of a wafer surface, in the original LOCOS form, always incurred lateral encroachment, or tapering of the field oxide into the active areas growing under the silicon nitride mask. This tapering effect, called "birdbeaking" is a sacrifice of active areas that becomes significant for feature sizes less than 1.5 microns. The active area becomes smaller than the initial dimensions of the nitride layer. Attempts to suppress birdbeaking such as forming thicker nitride layers, caused stress-related defects in the nearby substrate. Process complexity also increased substantially in attempting to avoid these stress-related defects. To achieve submicron geometries, there can be little or no physical loss of the active areas as occurs with the birdbeaking phenomenon.
To reduce the bird's beak effect, there has been proposed the use of a polysilicon layer between the nitride layer and the pad oxide layer as more fully described in U.S. Pat. No. 4,407,696, issued Oct. 4, 1983 to Han et al. The use of the polysilicon layer in the LOCOS process, known as poly-buffered LOCOS, is used to reduce oxidation induced stacking faults resulting from the stress caused by the different thermal coefficients of expansion between the silicon substrate and a thick silicon nitride layer overlying the substrate.
The field oxide layer grown using poly-buffered LOCOS thus comprises the oxide derived from the silicon substrate, a portion of the pad oxide layer and the oxide derived from the polysilicon layer. Afterwards, the nitride layer, the polysilicon layer and the pad oxide are etched at the edges of the field oxide. The poly-buffered LOCOS process reduced the bird's beak area over standard LOCOS resulting in less encroachment of the tapered portion of the field oxide into the active areas under the nitride mask. However, the bird's beak effect still remained, due to the oxidation of the polysilicon layer. In addition, the complexity of the process increased substantially in order to achieve the resulting structure.
In order to further decrease the bird's beak area using polybuffered LOCOS, the present invention uses a nitrogen implant into the polysilicon layer followed by an annealing step to encapsulate the polysilicon layer in silicon nitride. As with standard polybuffered LOCOS, the stress caused between the thick nitride layer over a thin pad oxide and the silicon substrate is reduced by the addition of the polysilicon layer. In addition, the bird's beak area is substantially reduced due to the encapsulation of the polysilicon in silicon nitride. The oxidation of the polysilicon is reduced or eliminated. None of the polysilicon layer thus forms any part of the thermally grown field oxide. The resulting bird's beak area under the nitride mask is thus substantially reduced.